Selecting a working fluid for a Rankine-cycle engine

For a specified power output, the required size and obtained performance of a thermodynamic power system releasing mechanical work are very dependent upon the properties of the working fluid. Organic fluids possess several advantages over water as the working fluid in a Rankine-cycle engine utilising low-grade energy as the heat input, particularly for low power output applications. In this study, the required characteristics of the [`]ideal' working fluid are discussed. These attributes cannot be satisfied simultaneously by any one fluid for all applications. With Rankine-cycle engines, operating between maximum and minimum temperature limits of 120°C and 40°C, respectively, the conclusions from investigating the suitabilities of sixtyeight potential working fluids are presented. The thermodynamic appropriateness, thermal stability, availability, cost and safety requirements were the primary factors to be satisfied in the screening process Three superior fluorinated hydrocarbons--R-11, R-113 and R-114--commonly used as refrigerants, were eventually short listed. A final assessment, based on the performances of these selected fluids, in the Rankine-cycle system of interest in this project, indicated that R-113 is the most suitable candidate for the application envisaged.